The present invention relates to capacitive touch-pad devices and, more particularly, to means for adjusting the output of a capacitive touch-pad device to match the threshold voltage of a comparator circuit used to sense the touch and no-touch conditions.
Known capacitive touch-pad devices have a pair of spatially-separated electrodes, commonly referred to as the transmitting electrode and the receiving electrode, fabricated upon a first surface of a dielectric layer, with a touch electrode fabricated upon the remaining surface of the dielectric layer and having an area enclosing the boundaries of the transmitting and receiving electrodes fabricated upon the opposite substrate surface. A source of a scanning voltage, commonly a voltage pulse, is coupled to the transmitting electrode and an input of a voltage comparator means is typically connected to the receiving electrode. A pair of series-connected capacitances (between the transmitting electrode and the touch electrode, and between the touch electrode and the receiving electrode) allow a certain proportion of the scan voltage to reach the voltage comparator input, when an object is not contacting or placed adjacent to the touch electrode; a different proportion of the scan voltage appears at the voltage comparator input when a relatively low impedance is placed between the touch electrode and a circuit ground. The comparator is configured to sense the change in signal amplitude at its input in the touch and in the no-touch conditions and to change the magnitude of its output accordingly.
The voltage comparator generally has a minimum threshold voltage which is typically greater in absolute value than the output signal from a capacitance touch-pad device which is activated by a relatively low scanning voltage, up to a magnitude of about 30 volts peak. Thus, the use of low scanning voltages, e.g. 30 volts peak or less, are not practical as the comparator input signal, in either the touch or no-touch condition is less than the minimum threshold voltage and the comparator remains in a single state in either condition.
It has been suggested to use direct current biasing techniques at the voltage comparator input to achieve an offset against the minimum threshold voltage and thereby reduce the threshold voltage to a value sufficiently small, to allow low amplitude scan voltages are usable. However, in the scanning mode, wherein excitation signals are applied to the transmitting electrode as a pulse occurring only at certain predetermined times, the voltage comparator input is typically reset to a fixed voltage level (to prevent false activation of the comparator output by noise and other extraneous signals occurring between scan pulses) and is released from the fixed voltage level immediately prior to the leading edge of the scan voltage pulse, whereby any D.C. bias introduced at the voltage comparator input would be rendered unusable by the comparator input reset circuit.
A dynamic biasing scheme which introduces a signal, offsetting the minimum threshold voltage, at the start of the scan voltage pulse will not be offset by the resetting of the comparator input and allows reduction in magnitude of the relatively high amplitude scan voltages presently utilized, thus reducing the cost of the electronics associated with the capacitive touch-pad device.